1. Field of the Invention
The present invention relates to a method of controlling communication between two nodes in a communication system and in particular, but not exclusively to controlling the data rate used in an enhanced dedicated channel in a UMTS system.
2. Description of the Related Art
A mobile communication system is an example of a system in which an access network is provided to allow access to the system functionality for user terminals.
In a universal mobile telecommunication system (UMTS), a radio access network typically provides access for user equipment to a mobile communications system core network. The user equipment typically communicates with the access network over a radio interface, the access network including a plurality of Node Bs or base transceiver stations with which the user equipment establishes a connection. Each of the Node Bs is connected to one or more radio network controllers.
A dedicated channel (DCH) is provided in a UMTS system for uplink traffic from the user equipment to the radio network controller via the Node B. In the third generation partnership project, technical specification group radio access network (3GPP TSG-RAN) there has been proposed high speed uplink packet access, also known in 3GPP as frequency division duplex (FDD) enhanced uplink, including an enhanced DCH, E-DCH. This proposal is documented in 3GPP TR 25.896.
Currently, it is proposed to distribute some of the packet scheduler functionalities of the radio network controller to the Node Bs. This is to provide faster scheduling of bursty non real time traffic than the layer 3 in the RNC radio network controller facilitates. The idea is that with faster link adaptation, it is possible to more efficiently share the up link power resource between packet data users. When packets have been transmitted from one user, the scheduled resource can be made immediately available to another user. This avoids the peak variability of noise rise, when high data rates have been allocated to users running bursty high data rate applications.
With current proposals, the packet scheduler is located in the RNC and is therefore limited in its ability to adapt to the instantaneous traffic. This is because of bandwidth constraints on the RRC (radio resource control) signalling interface between the RNC and the user equipment. To accommodate the variability, the packet scheduler must be conservative in allocating up link power to take into account the influence from inactive users in the following scheduling period. This proposal is spectrally inefficient for high allocated data rates and long release time of values.
There are also current proposals which have the Node B taking care of the allocated up link resources with EDCH. For the transmission of data, it is proposed that the user equipment selects a transport format combination TFC that suits the amount of data to be transmitted in its RLC radio link control buffer, subject to constraints on the maximum transmission power of the user equipment and the maximum allowed TFC. If needed, the user equipment can request a high bit rate and the Node B can decide whether or not to grant additional resources. The Node B may also adjust the resources allocated to all user equipment according to the cell load.
Signalling is required to support the Node B scheduling. For the up link, the user equipment requests and/or status needs to be transmitted to Node B. For example, the user equipment tells Node B that it would like additional resources to be allocated or the user equipment signals to Node B how much data it has in its buffer so that the Node B can assess how much resource it needs. As far as the downlink is concerned, the Node B needs to send scheduling commands, for example, grant additional resources.
For downlink, two signalling methods are possible. One known method is for the Node B to directly signal to the user equipment, the index of the highest TFC allowed (hereinafter referred to as TFCmax).
An alternative method is a step wise method where the Node B signals UP/DOWN/KEEP commands for changing the index of the highest TFC allowed. This signalling is called rate grant (RG) and its size can be as low as one bit (an UP command and a DOWN command only) or two bits (an UP command, a DOWN command and a KEEP command).
The absolute signalling method requires a larger signalling overhead compared to the stepwise method. The step wise signalling method can prevent an sudden increase of noise but if a large amount of data abruptly occurs in the user equipment while transmitting at a low data rate, the step wise rate increase will result in long ramp up times to reach the required data rate. Whilst this is not a particular issue when the cell load is high, it is one when the cell load is low. This is because the user equipment has to wait for a long time although it could have been allowed to use the high data rate right away. Additionally, when the cell load becomes critical it may be beneficial to quickly decrease the allocated resources. The step wise rate decrease does not allow this.
Since packet data applications are typically bursty in nature with large variations in their resource requirements, adjusting the user equipment data rate quickly can increase the overall system performance especially when the cell's load is low.
In the 3GPP document R1 040683—enhanced uplink scheduling it is proposed to use a “busy indicator” to indicate whether the cell is fully loaded or not. This is in combination with an indication of the maximum resource the user equipment is allowed to consume in the uplink. If the busy indicator is not set, the user equipment may use resources up to the maximum resource indication transmitted by the cell. On the other hand, if the busy indicator is set, the cell is fully loaded and the user equipment is not allowed to start transmitting using resources higher than a lower resource limit.
It has also been proposed by the present applicant in U.S. Ser. No. 10/764,143 filed 23 January that when the user equipment initial data rate is low or zero, the downlink scheduling related signalling is interpreted differently or even signalled differently than when the user equipment initial data rate is higher. The method proposed has the following steps:
Initially, the user equipments allowed data rate is low—low being a predetermined TFC or anything below that.
The user equipment asks for a higher data rate by sending a “data rate increment request” bit to the Node B.
If the node accepts the request, it responds with “data rate grant up” bit. The downlink signalling is interpreted differently by the user equipment due to the initial low data rate than it would be if the data rate was higher i.e. the resulting data rate can be more than one step higher than the initial data was. This how much “more than one step” could for example be signalled by a higher layers when setting up the connection.